Oral Tumors and Their Biology
Stephen Withrow United States
INCIDENCE AND RISK FACTORS
Oral cancer accounts for 6% of canine cancer and 3% of all cancers in cats. A general summary of the common oral tumors is found in Table 1.
PATHOLOGY AND NATURAL BEHAVIOR
Oral fibrosarcoma will often look surprisingly benign histologically. If the cancer in question is growing rapidly, is recurrent, or is invading bone, however, the clinician should dictate treatment as for malignant cancer. Fibrosarcoma is very invasive locally but metastasizes in less than 20% of cases (usually to the lungs).
Malignant melanoma can present a confusing histopathologic picture if the tumor or the biopsy section does not contain melanin (one-third of all cases). A histopathologic diagnosis of “undifferentiated sarcoma” should be looked upon with suspicion for possible underlying melanoma. Melanoma has a predilection to metastasize to regional lymph nodes and then lung.
Squamous cell carcinoma is usually a straightforward histologic diagnosis. Severe and extensive involvement of bone is common in the cat. The metastatic rate in the cat is somewhat unknown since so few cats have their local disease controlled to observe the long-term metastatic potential. Metastasis in the canine is very site-dependent with the rostral oral cavity having a low metastatic rate and the caudal tongue and tonsil having a high metastatic potential.
The “traditional” epulides are similar to gingival hyperplasia in appearance and are usually confined to one or two sites at the gum margin. They are slow growing, firm, and generally covered by intact epithelium. These are classified as fibrous epulides or ossifying epulides depending on the presence or absence of bone. A third class of epulides recently has been termed acanthomatous epulis instead of the previous term of adamantinoma. Some pathologists use the terms interchangeably. These are much more locally invasive and virtually always invade bone although they do not metastasize.
DIAGNOSTIC TECHNIQUES AND WORK-UP
The diagnostic evaluation for oral cancers is critical due to the wide ranges of cancer behavior and therapeutic options available. The most likely cancers to have positive chest radiographs at the time of diagnosis are melanoma and squamous cell carcinoma of the caudal oral and pharyngeal area.
Cancers that are adherent to bone, other than simple epulides, should have regional radiographs taken under anesthesia. When 40% or more of the cortex is destroyed, lysis may be observed. However, apparently normal radiographs do not rule out bone invasion. Regional lymph nodes should be carefully palpated for enlargement or asymmetry. When palpable, they should be aspirated.
The last step, under the same anesthesia, is careful palpation and a large incisional biopsy. Large samples of healthy tissue at the edge and center of the lesion will increase the diagnostic yield. The biopsy site should be located in such a position as to be easily included in a possible resection. For small lesions, excisional biopsy may be undertaken at the time of initial evaluation. For more extensive disease, waiting for biopsy results to accurately plan treatment is encouraged.
The prognosis for acanthomatous epulis/adamantinoma is excellent with surgery and/or irradiation with local recurrence rates less than 5%.
The outlook for squamous cell carcinoma is very site and species dependent. Cancers of the canine in the rostral mouth are curable with surgery or irradiation, while those of tonsil or base of the tongue, are highly metastatic and likely to recur locally or regionally. Local control of feline squamous cell carcinoma is poor with either surgery or radiation therapy.
Overall, over 25% of dogs with oral malignant melanomas will survive one year or more. Dogs with tumors less than two centimeters in diameter have a median survival of 511 days as opposed to dogs with lymph node involvement or tumors greater than two centimeters in diameter whose median survival is 164 days. Recurrent malignant melanoma does worse than primarily treated disease that achieves permanent local control. Age, breed, sex, degree of pigmentation, microscopic appearance (?), and anatomic site are not prognostic.
Local control of fibrosarcoma is more of a problem than metastasis. The best one-year survivals with almost any treatment are no better than 25-40%. Fibrosarcomas are generally considered radiation resistant for measurable disease but control rates are improved when treating microscopic postoperative residual disease.
TABLE 1: Summary of Common Oral Cancers of the Dog and Cat
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